  The inelastic quasiparticle lifetime due to the electron-electron interaction
(out-scattering time in the kinetic equation formalism) is calculated for
finite metallic diffusive systems (quantum dots) in the whole range of
parameters. Both cases of ``continuous'' (the inelastic level broadening much
exceeds the mean level spacing) and ``discrete'' spectrum are analyzed. In
particular, crossover between one- and zero-dimensional regimes is studied in
detail. In the case of continuous spectrum the out-scattering time is shown to
be the same as the inelastic time entering expressions for universal
conductance fluctuations and persistent currents. It is also found to be
shorter than the phase-breaking time in two- and one-dimensional systems, while
in zero-dimensional systems these two times coincide. In the case of discrete
spectrum for small enough systems a universal behavior of the scattering time
is obtained. For temperatures below the mean level spacing the out-scattering
rate is shown to be vanishingly small.
