  The spin-Hall conductivity in spatially-homogeneous two-dimensional electron
systems described by the spin-orbit Hamiltonian \hbar \Omega_p \sigma is
presented as a sum of the universal part Me/8 \pi \hbar determined by the Berry
phase \Phi=M \pi (M is an odd integer, the winding number of the vector
\Omega_p) and a non-universal part which vanishes under certain conditions
determined by the analytical properties of \Omega_p. The analysis reveals a
rich and complicated behavior of the spin-Hall conductivity which is relevant
to both electron and hole states in quantum wells and can be detected in
experiments.
