  Surface-state contributions to the dc conductivity of most homogeneous metals
exposed to uniform electric fields are usually as small as the system size is
large compared to the lattice constant. In this work, we show that surface
states of topological metals can contribute with the same order of magnitude as
the bulk even in large systems. This effect is intimately related to the
intrinsic anomalous Hall effect, in which an applied voltage induces chiral
surface-state currents proportional to the system size. Unlike the anomalous
Hall effect, the large contribution of surface states to the dc conductivity is
also present in time-reversal invariant Weyl semimetals, where the surface
states come in counter-propagating time-reversed pairs. While the Hall voltage
vanishes in the presence of time-reversal symmetry, the twinned chiral surface
currents develop similarly as in the time-reversal broken case. For this effect
to occur, the relaxation length associated with scattering between
time-reversed partner states needs to be larger than the separation of
contributing surfaces, which results in a characteristic size dependence of the
resistivity and a highly inhomogeneous current-density profile across the
sample.
