Ballistic electron transport and two-level resistance fluctuations in noble-metal nanobridges

The electrical properties of microfabricated nanobridges of copper, silver, and gold with contact diameters in the range 4--32 nm have been studied. High-quality point-contact spectra are evidence that electron transport is ballistic in these nanobridges. A comparison of our spectra with spectra from mechanical point contacts shows that microfabricated nanobridges are at least as good as mechanical point contacts for study of the electron-phonon interaction. Further, in Au nanobridges we have observed defect motion induced two-level resistance fluctuations (TLF's). An expression is derived for the voltage dependence of the temperature Td of a defect in a nanobridge at low lattice temperatures. Using this expression for Td, the experimental voltage dependence of the TLF's is successfully described by a thermal-activation model for the fluctuation rates, in which the voltage dependence of the activation energy and defect temperature is included. The values for the attempt time, activation energy, and electromigration parameter are as expected for defects in metals. An analysis of the two TLF's studied, showing a striking difference in both voltage dependence and magnitude of the duty cycle, suggests that rearrangement of complex defects is the mechanism behind the TLF behavior.