Conductance fluctuations in a ballistic metallic point contact

The magnetoconductance of a 3D ballistic Ag point contact is found to show reproducible fluctuations with rms amplitude 2 orders of magnitude lower than usually observed in diffusive systems and insensitive to temperature up to T=5 K. This is explained by quantum interference of electrons, the length scale of the trajectories of interfering waves being the elastic mean free path. For an applied voltage across the constriction, the observed magnetofingerprint changes asymmetrically under bias reversal with a roughly constant rms amplitude. At higher voltages (V\ensuremath{\ge}10 mV), electron-phonon coupling destroys the phase coherence and a strong decrease in fluctuation amplitude is observed.