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 ${\mathit{E}}_{\mathit{c}}$ 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 ${\mathit{E}}_{\mathit{c}}$≊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 ${\mathit{E}}_{\mathit{c}}$/${\mathit{k}}_{\mathit{B}}$ or for ac modulation voltages exceeding ${\mathit{E}}_{\mathit{c}}$/e, a decrease of the fluctuation amplitude is observed, in agreement with averaging over uncorrelated fluctuation patterns. However, for dc bias voltages larger than ${\mathit{E}}_{\mathit{c}}$/e the amplitude of fluctuations of the differential conductance remains unchanged for voltages up to V≊8 mV. This behavior is in strong contrast to observations and theoretical descriptions of conductance fluctuations in diffusive metallic samples.