  We point out that even at the absolute zero of temperature environmental
decoherence limits the destructive interference between time-reversed paths for
an electron in a disordered metal, and thus causes the leading (`weak
localization') quantum correction to the conductivity to saturate at T=0. Our
calculation, which is intended to be illustrative rather than complete, uses a
model in which an electron interacts with the fluctuations of the mean voltage
in the sample. The average of the fluctuations produces the steady damping well
known in Brownian motion, introduces a direction of time, ensures that
arbitrarily long time-reversed electron paths lose phase coherence, and is
consistent with the experimental observation of a finite low temperature
phase-breaking lifetime.
