  We have set up a set of many-body kinetic Bloch equations with spacial
inhomogeneity. We reexamined the widely adopted quasi-independent electron
model and showed the inadequacy of this model in studying the spin transport.
We further pointed out a new decoherence effect based on interference effect
along the diffusion in spin transport problem due to the so called
inhomogeneous broadening effect in the Bloch equations. We have shown that this
inhomogeneous broadening effect can cause the spin decoherence alone even
without the scattering and that the resulting decoherence is more important
than the dephasing effect due to the D'yakonov-Perel' (DP) term together with
the scatterings.
