  HgTe quantum wells and surfaces of three-dimensional topological insulators
support Dirac fermions with a single-valley band dispersion. In the presence of
disorder they experience weak antilocalization, which has been observed in
recent transport experiments. In this work we conduct a comparative theoretical
study of the weak antilocalization in HgTe quantum wells and topological
surface states. The difference between these two single-valley systems comes
from a finite band gap (effective Dirac mass) in HgTe quantum wells in contrast
to gapless (massless) surface states in topological insulators. The finite
effective Dirac mass implies a broken internal symmetry, leading to suppression
of the weak antilocalization in HgTe quantum wells at times larger than certain
t_M, inversely proportional to the Dirac mass. This corresponds to the opening
of a relaxation gap 1/t_M in the Cooperon diffusion mode which we obtain from
the Bethe-Salpeter equation including relevant spin degrees of freedom. We
demonstrate that the relaxation gap exhibits an interesting nonmonotonic
dependence on both carrier density and band gap, vanishing at a certain
combination of these parameters. The weak-antilocalization conductivity
reflects this nonmonotonic behavior which is unique to HgTe QWs and absent for
topological surface states. On the other hand, the topological surface states
exhibit specific weak-antilocalization magnetoconductivity in a parallel
magnetic field due to their exponential decay in the bulk.
