  We point out that the low temperature saturation of the electron phase
decoherence time in a disordered conductor can be explained within the existing
theory of weak localization provided the effect of quantum (high frequency)
fluctuations is taken into account. Making use of the fluctuation-dissipation
theorem we evaluate the quantum decoherence time, the crossover temperature
below which thermal effects become unimportant, and the weak localization
correction $\delta \sigma$ at T=0. For 1d systems the latter is found to be
$\delta \sigma \propto 1/ \sqrt{N}$, where $N$ is the number of conducting
channels.
