  We present a general unifying theory for spin polarization decay due to the
interplay of spin precession and momentum scattering that is applicable to both
spin-1/2 electrons and spin-3/2 holes. Our theory allows us to identify and
characterize a wide range of qualitatively different regimes. For strong
momentum scattering or slow spin precession we recover the D'yakonov-Perel
result, according to which the spin relaxation time is inversely proportional
to the momentum relaxation time. On the other hand, we find that, in the
ballistic regime the carrier spin polarization shows a very different
qualitative behavior. In systems with isotropic spin splitting the spin
polarization can oscillate indefinitely, while in systems with anisotropic spin
splitting the spin polarization is reduced by spin dephasing, which is
non-exponential and may result in an incomplete decay of the spin polarization.
For weak momentum scattering or fast spin precession, the oscillations or
non-exponential spin dephasing are modulated by an exponential envelope
proportional to the momentum relaxation time. Nevertheless, even in this case
in certain systems a fraction of the spin polarization may survive at long
times. Finally it is shown that, despite the qualitatively different nature of
spin precession in the valence band, spin polarization decay in spin-3/2 hole
systems has many similarities to its counterpart in spin-1/2 electron systems.
