CORRELATION-ENERGY OF CONDUCTANCE FLUCTUATIONS IN BALLISTIC SILVER POINT CONTACTS

We have studied conductance fluctuations of microfabricated silver point contacts, which operate in the ballistic electron-transport regime. The correlation energy E(c) of the conductance fluctuations has been determined from the voltage correlation of the fluctuations, and from the temperature and modulation voltage dependence of the fluctuation amplitude. For all the devices investigated E(c) almost-equal-to 1 meV. This value is in good agreement with a model where the conductance fluctuations are due to quantum interference of electrons backscattered by a few impurities at a distance of the order of the elastic mean free path from the constriction. For temperatures higher than E(c)/k(B) or for ac modulation voltages exceeding E(c)/e, a decrease of the fluctuation amplitude is observed, in agreement with averaging over uncorrelated fluctuation patterns. However, for dc bias voltages larger than E(c)/e the amplitude of fluctuations of the differential conductance remains unchanged for voltages up to V almost-equal-to 8 mV. This behavior is in strong contrast to observations and theoretical descriptions of conductance fluctuations in diffusive metallic samples.