The conductance of a point contact between two crystals of metallic ͑La 0.75 Sr 0.25 ͒MnO 3 shows steps as the contact is broken. A histogram of the conductance values observed in 60 breaks exhibits a series of sharp peaks at integer multiples of G 0 ϭ2e 2 /h. Quantum conductance in these ceramics is qualitatively different from that normally seen in metals and suggests that no neck formation occurs during fracture of a contact between these brittle materials. ͓S0163-1829͑98͒02731-3͔The phenomenon of quantum conductance was observed in 1988 using an electrostatically defined constriction in a two-dimensional electron gas in a semiconductor. 1,2 Since then, quantum conduction has been explored in metal nanowires using a variety of experimental techniques, some of which are quite simple. A clear observation of quantum steps in metals was obtained by Muller, van Ruitenbeek, and de Jongh 3 using mechanically controllable break junctions. The scanning tunneling microscope ͑STM͒ has proved to be a convenient tool for producing nanowires in various conditions ͑low temperature, 4,5 room temperature, 6,7 high vacuum͒. 9 The quantum steps have been reported in various metals ͓Au, 4-7,9-12 Al, 13 Pb, 4 Cu, 8,[11][12][13]8,[10][11][12][13] Ni, 8-10 ͔ in liquid metals ͓Hg and Sb ͑Ref. 14͔͒, in narrow-gap semiconductors ͓Pb 1Ϫx Sr x Se ͑Ref. 15͔͒ and recently in a semi-metal ͓Bi ͑Ref. 16͔͒. Quantized conductance arises from ballistic electron transport in a narrow constriction produced as the contact between two electrodes is broken. The conduction is then independent of material properties, and depends only on the geometry of the neck. The quantization of the conductance follows from the quantization of the electron momentum in the directions perpendicular to the direction of the current flow. The Landauer-Buttiker formalism 17-19 leads to a conductance G of the formwhere T i is the probability that an electron entering a conduction channel is transmitted. In the case of very narrow constrictions ͑around 1 nm͒ the splitting between the conduction channels is of the order of 1 eV. Thus it is possible to observe the effect at room temperature.Here we compare the contact-breaking behavior of a ferromagnetic metal ͑Fe͒ and a ferromagnetic oxide ͑La 0.75 Sr 0.25 ͒MnO 3 . Nothing in the data appears to reflect the ferromagnetic character of the samples, but we identify clear qualitative differences between the metal and the ceramic. We denote the quantum of conductance 2e 2 /h as G 0 .Most of the studies on quantum conductance have been performed using STM tips or with a constriction in twodimensional ͑2-D͒ electron gas in a semiconductor. Another technique for creating nanowires is to break a metal wire to create two fresh electrodes and then mechanically bring them back into contact. 13,20 We have adopted the very simple method proposed by 11 in which two macroscopic samples ͑usually wires͒ are brought into contact and then pulled apart while continuously monitoring the resistance of the contact. The contact was biased with 36 mV...

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