  The retardation and temperature effects in two-body collisions are studied.
The collision integral with retardation effects is obtained on the base of the
Kadanoff- Baym equations for Green functions in a form with allowance for
reaching the local equilibrium system. The collisional relaxation times of
collective vibrations are calculated using both the transport approach and
doorway state mechanism with hierarchy of particle-hole configurations in
heated nuclei. The relaxation times of the kinetic method are rather slowly
dependent on multipolarity of the Fermi surface distortion and mode of the
collective motion. The dependence of the relaxation times on temperature as
well as on frequency of collective vibrations is considered and compared. It is
shown that variations of the in-medium two-body cross-sections with energy lead
to non-quadratic dependence of the collisional relaxation time both on
temperature and on collective motion frequency.
