  We study the effect of contact hyperfine interaction on the nuclear spin
diffusion coefficients in semiconductor quantum dots. The diffusion
coefficients are calculated with both the method of moment and density matrix.
We show that nuclear spin diffusion is strongly suppressed by the nonuniform
hyperfine coupling resulting from the confined electron wavefunction. Our
results agree with the observed suppression of nuclear spin diffusion in these
structures in recent experiments, and clarify the degree of validity of the
method of moment in an inhomogeneous system.
