  We calculate the temperature dependence of the weak localization correction
in a two dimensional system at arbitrary relation between temperature, $T$ and
the elastic mean free time. We describe the crossover in the dephasing time
${\tau_\phi(T)}$ between the high temperature, $1/\tau_\phi \simeq T^2 \ln T$,
and the low temperature $1/\tau_\phi \simeq T$ behaviors. The prefactors in
these dependences are not universal, but are determined by the Fermi liquid
constant characterising the spin exchange interaction.
