  Electronic quantum effects in disordered conductors are controlled by the
dephasing rate of conduction electrons. This rate is expected to vanish with
the temperature. We consider the very intriguing recently reported apparent
saturation of this dephasing rate in several systems at very low temperatures.
We show that the ``standard model'' of a conductor with static defects can {\em
not} have such an effect. However, allowing some dynamics of the defects may
produce it.
