  With an isospin- and momentum-dependent transport model, we find that the
degree of isospin diffusion in heavy ion collisions at intermediate energies is
affected by both the stiffness of the nuclear symmetry energy and the momentum
dependence of the nucleon potential. Using a momentum dependence derived from
the Gogny effective interaction, recent experimental data from NSCL/MSU on
isospin diffusion are shown to be consistent with a nuclear symmetry energy
given by $E_{\text{sym}}(\rho)\approx 31.6(\rho /\rho_{0})^{1.05} $ at
subnormal densities. This leads to a significantly constrained value of about
-550 MeV for the isospin-dependent part of the isobaric incompressibility of
isospin asymmetric nuclear matter.
