  Conduction electrons in disordered metals and heavily doped semiconductors at
low temperatures preserve their phase coherence for a long time: phase
relaxation time $\tau_\phi$ can be orders of magnitude longer than the momentum
relaxation time. The large difference in these time scales gives rise to well
known effects of weak localization, such as anomalous magnetoresistance. Among
other interesting characteristics, study of these effects provide quantitative
information on the dephasing rate $1/\tau_\phi$. This parameter is of
fundamental interest: the relation between $\hbar/\tau_\phi$ and the
temperature $T$ (a typical energy scale of an electron) determines how well a
single electron state is defined. We will discuss the basic physical meaning of
$1/\tau_\phi$ in different situations and its difference from the energy
relaxation rate. At low temperatures, the phase relaxation rate is governed by
collisions between electrons. We will review existing theories of dephasing by
these collisions or (which is the same) by electric noise inside the sample. We
also discuss recent experiments on the magnetoresistance of 1D systems: some of
them show saturation of $1/\tau_\phi$ at low temperatures, the other do not. To
resolve this contradiction we discuss dephasing by an external microwave field
and by nonequilibrium electric noise.
