We discuss the use of mechanically controllable break-junctions (MCBs) as electrodes for contacting individual objects such as clusters or molecules. A big advantage of MCB electrodes is the possibility of varying the contact geometry and, accordingly, the contact resistances and the quantum coherent transport properties between the object under study and the electrodes. We compare the suitability of single-atom contacts of different elements as electrode material. Finally, we present preliminary results on electronic transport through individual (or a few) fullerene molecules contacted by gold MCB electrodes. The typical resistances are in the order of the quantum of conductance. The circuits sustain voltages of the order of 1 V and are mechanically more stable than single-atom contacts between the gold electrodes alone.

We report low-temperature measurements of the electrical transport properties of atomic contacts of the superconducting metal zinc, arranged with lithographically fabricated mechanically controllable breakjunctions ͑MCB͒ as well as with MCBs made from whiskers. The conductance histogram shows several peaks-not regularly observed for multivalent metals. The first peak-corresponding to the single-atom contact-is located slightly below one conductance quantum and is split into two subpeaks, indicating two preferred configurations of the single-atom contact. The existence of two configurations can also be traced by analyzing individual conductance vs distance curves of the MCB. The current-voltage characteristics in the superconducting state show nonlinearities due to multiple Andreev reflections ͑MAR͒, which we use for determining the transport channels. With the help of a tight-binding model for the transport channels we establish a correlation between the preferred conductance values and preferred atomic configurations.

We report low-temperature measurements of the electrical transport properties of single-atom and tunnel contacts of Zn. We present a variant of the mechanically controllable break-junction technique that comprises a whisker as the central part. This technique combines the possibility to fabricate reproducibly single-atom contacts and well-characterized electronic and lattice properties of the electrodes. From tunneling current-voltage characteristics, we deduce a well-pronounced quasiparticle spectrum for whiskers, while for standard "notched-wire" and thin-film break junctions the spectrum is smeared out. The tunneling density of states 1 is usually deduced from electronic transport measurements through tunneling contacts , fabricated by different methods. However, although discovered many years ago, for several elementary supercon-ductors direct transport measurements are still lacking because of the difficulty to fabricate suitable tunneling contacts. The divalent metal Zn is an example for this situation where, besides the metallurgical problems that hinder the creation of tunneling contacts with standard methods, the superconducting properties are anisotropic. Single-atom or few-atom contacts are model systems to investigate the electronic transport properties on the atomic scale because the electronic conduction can be regarded as a fully quantum-mechanical scattering problem, i.e., the con-ductance G can be attributed to independent nicknamed conduction channels which contribute to the total conduc-tance with a weight given by their transmission coefficients i : G = G 0 i=1 N i G 0 =2e 2 / h is the conductance quantum and N the number of channels. 2 An atomic-size contact between two metallic electrodes can accommodate only a small number of conduction channels the number and i of which depend on the chemical properties of the atoms forming the contact and on their geometrical arrangement. The latter information has been deduced from the fact that the transport properties change when rearranging the contact region. 3,4 The use of a hexagonal metal allows to establish different configurations of the few-atom contacts in contrast to the cubic metals that have been studied so far. In principle, all physical properties that depend on the channel ensemble i could be used to determine the transport channels. However, independent determination of all i s with arbitrary values has so far only successfully been performed by analyzing current-voltage characteristics I-Vs of superconducting contacts. The analysis relies on the quantitative agreement between the calculated I-Vs for BCS super-conductors and the experimental data. 4,5 Consequently, the superconducting properties of the electrodes forming the atomic-size contact have to be of high quality. In the present work the measurement of the i s is accomplished for Zn, a metal not explored so far, by using mechanically controllable break junctions MCB Refs. 3 and 6 of whiskers glued on top of a prepatterned flexible substrate see Fig. 1. Zn whiskers grow with a p...