  We report a novel phenomenon that the universal conductance fluctuations
(UCF) can be suppressed by a small electric field E. The experiment has been
carried out on single crystals of Si doped heavily with P and B beyond the
critical composition of insulator to metal transition. The phenomenon is
identified as a consequence of electric field induced dephasing of the electron
wavefunction. Over the range of measurements, the observed dephasing rate
(\tau_\phi^{-1}) varied as \tau_\phi^{-1} = aT + bE^q with q \approx 1.3 and
for E >> E^*, a cross-over field, \tau_\phi^{-1} \sim E^q, independent of T.
This experiment also establishes that the UCF can be utilized as a sensitive
electron "interferometer" to measure dephasing rate.
