  Topological semimetals have been at the forefront of experimental and
theoretical attention in condensed matter physics. Among these, recently
discovered Weyl semimetals have a dispersion described by a three-dimensional
Dirac cone, which is at the root of exotic physics such as the chiral anomaly
in magnetotransport. In a time reversal symmetric (TRS) Weyl semimetal film,
the confinement gap gives the quasiparticles a mass, while TRS is preserved by
having an even number of valleys with opposite masses. The film can be tuned
through a topological phase transition by a gate electric field. In this work,
we present a theoretical study of the quantum corrections to the conductivity
of a topological semimetal thin film, which is governed by the complex
interplay of the chiral band structure, mass term, and scalar and spin-orbit
scattering. We study scalar and spin-orbit scattering mechanisms on the same
footing, demonstrating that they have a strong qualitative and quantitative
impact on the conductivity correction. We show that, due to the spin structure
of the matrix Green's functions, terms linear in the extrinsic spin-orbit
scattering are present in the Bloch and momentum relaxation times, whereas
previous works had identified corrections starting from the second order. In
the limit of small quasiparticle mass, the terms linear in the impurity
spin-orbit coupling lead to a potentially observable density dependence in the
weak antilocalization correction. Moreover, when the mass term is around 30
percent of the linear Dirac terms, the system is in the unitary symmetry class
with zero quantum correction and switching the extrinsic spin-orbit scattering
drives the system to the weak antilocalization. We discuss the crossover
between the weak localization and weak antilocalization regimes in terms of the
singlet and triplet Cooperon channels, tuning the spin-orbit scattering
strength.
