  In this talk, we first briefly review the isospin dependence of the total
nucleon effective mass $M^{\ast}_{J}$ inferred from analyzing nucleon-nucleus
scattering data within an isospin dependent non-relativistic optical potential
model, and the isospin dependence of the nucleon E-mass $M^{\ast,\rm{E}}_{J}$
obtained from applying the Migdal-Luttinger theorem to a phenomenological
single-nucleon momentum distribution in nuclei constrained by recent
electron-nucleus scattering experiments. Combining information about the
isospin dependence of both the nucleon total effective mass and E-mass, we then
infer the isospin dependence of nucleon k-mass using the well-known relation
$M^{\ast}_{J}=M^{\ast,\rm{E}}_{J}\cdot M^{\ast,\rm{k}}_{J}$. Implications of
the results on the nucleon mean free path (MFP) in neutron-rich matter are
discussed.
