  Diffusion of electrons in two-dimensional disordered systems with spin-orbit
interactions is investigated numerically. Asymptotic behaviors of the second
moment of the wave packet and of the temporal auto-correlation function are
examined. At the critical point, the auto-correlation function exhibits the
power-law decay with a non-conventional exponent $\alpha$ which is related to
the fractal structure in the energy spectrum and in the wave functions. In the
metallic regime, the present results imply that transport properties can be
described by the diffusion equation for normal metals.

Condensed Matter