  We study the quantum dynamics of interstitials and vacancies in a
two-dimensional Wigner crystal (WC) using a semi-classical instanton method
that is asymptotically exact at low density, i.e., in the $r_s\to \infty$
limit. The dynamics of these point defects mediates magnetism with much higher
energy scales than the exchange energies of the pure WC. Via exact
diagonalization of the derived effective Hamiltonians in the single-defect
sectors, we find the dynamical corrections to the defect energies. The
resulting expression for the interstitial (vacancy) energy extrapolates to 0 at
$r_s = r_{\rm mit} \approx 70$ ($r_s \approx 30$), suggestive of a self-doping
instability to a partially melted WC for some range of $r_s$ below $r_{\rm
mit}$. We thus propose a "metallic electron crystal'' phase of the
two-dimensional electron gas at intermediate densities between a low density
insulating WC and a high density Fermi fluid.
