  The non-Markovian collision integral is obtained on the base of the
Kadanoff-Baym equations for Green functions in a form with allowance for small
retardation effects. The collisional relaxation times and damping width of the
giant isovector dipole resonances in nuclear matter are calculated. For an
infinite Fermi liquid the dependence of the relaxation times on the collective
vibration frequency and the temperature corresponds to the Landau's
prescription.
