We have developed a fast, yet highly reproducible method to fabricate metallic electrodes with nanometer separation using electromigration (EM). We employ four-terminal instead of two-terminal devices in combination with an analog feedback to maintain the voltage U over the junction constant. After the initialization phase (U 0.2 V), during which the temperature T increases by 80 − 150 o C, EM sets in shrinking the wire locally. This quickly leads to a transition from the diffusive to a quasiballistic regime (0.2 V U 0.6 V). At the end of this second regime, a gap forms (U 0.6 V). Remarkably, controlled electromigration is still possible in the quasi-ballistic regime.

We have studied the 1 / f voltage noise of gold nanocontacts in electromigrated and mechanically controlled break junctions having resistance values R that can be tuned from 10 ⍀ ͑many channels͒ to 10 k⍀ ͑singleatom contact͒. The noise is caused by resistance fluctuations as evidenced by the S V ϰ V 2 dependence of the power-spectral density S V on the applied dc voltage V. As a function of R the normalized noise S V / V 2 shows a pronounced crossover from ϰR 3 for low-Ohmic junctions to ϰR 1.5 for high-Ohmic ones. The measured powers of 3 and 1.5 are in agreement with 1 / f noise generated in the bulk and reflect the transition from diffusive to ballistic transport.