  In insulators, Born effective charges describe the electrical polarization
induced by the displacement of individual atomic sublattices. Such a physical
property is at first sight irrelevant for metals and doped semiconductors,
where the macroscopic polarization is ill-defined. Here we show that, in clean
conductors, going beyond the adiabatic approximation results in nonadiabatic
Born effective charges that are well defined in the low-frequency limit. In
addition, we find that the sublattice sum of the nonadiabatic Born effective
charges does not vanish as it does in the insulating case, but instead is
proportional to the Drude weight. We demonstrate these formal results with
density functional perturbation theory calculations of Al, and electron-doped
SnS$_2$ and SrTiO$_3$.
