  The Hikami-Larkin-Nagaoka (HLN) formula [Prog. Theor. Phys. 63, 707 (1980)]
describes the quantum corrections to the magnetoconductivity of a quasi-2D
electron gas (quasi-2DEG) with parabolic dispersion. It predicts a crossover
from weak localization to antilocalization as a function of the strength of
scattering off spin-orbit impurities. Here, we derive the conductivity
correction for massless Dirac fermions in 3D topological insulators (TIs) in
the presence of spin-orbit impurities. We show that this correction is always
positive and therefore we predict weak antilocalization for every value of the
spin-orbit disorder. Furthermore, the correction to the diffusion constant is
surprisingly linear in the strength of the impurity spin-orbit. Our results
call for a reinterpretation of experimental fits for the magnetoconductivity of
3D TIs which have so far used the standard HLN formula.
