  In the inner crust of a neutron star, at densities above the ``drip''
threshold, unbound ``conduction'' neutrons can move freely past through the
ionic lattice formed by the nuclei. The relative current density $n^i= n \bar
v^i$ of such conduction neutrons will be related to the corresponding mean
particle momentum $p_i$ by a proportionality relation of the form $n^i= {\cal
K}p^i$ in terms of a physically well defined mobility coefficient $\cal K$
whose value in this context has not been calculated before. Using methods from
ordinary solid state and nuclear physics, a simple quantum mechanical treatment
based on the independent particle approximation, is used here to formulate
$\cal K$ as the phase space integral of the relevant group velocity over the
neutron Fermi surface. The result can be described as an ``entrainment'' that
changes the ordinary neutron mass m to a macroscopic effective mass per neutron
that will be given -- subject to adoption of a convention specifying the
precise number density n of the neutrons that are considered to be ``free'' --
by $m_\star=n/{\cal K}$. The numerical evaluation of the mobility coefficient
is carried out for nuclear configurations of the ``lasagna'' and ``spaghetti''
type that may be relevant at the base of the crust. Extrapolation to the middle
layers of the inner crust leads to the unexpected prediction that $m_\star$
will become very large compared with m.
