  Localization properties of the doped Z_2-topological insulator are studied by
weak localization theory. The disordered Kane-Mele model for graphene is taken
as a prototype, and analyzed with attention to effects of the topological mass
term, inter-valley scattering, and the Rashba spin-orbit interaction. The known
tendency of graphene to anti-localize in the absence of inter-valley scattering
between K and K' points is naturally placed as the massless limit of Kane-Mele
model. The latter is shown to have a unitary behavior even in the absence of
magnetic field due to the topological mass term. When inter-valley scattering
is introduced, the topological mass term leaves the system in the unitary
class, whereas the ordinary mass term, which appears if A and B sublattices are
inequivalent, turns the system to weak localization. The Rashba spin-orbit
interaction in the presence of K-K' scattering drive the system to weak
anti-localization in sharp contrast to the ideal graphene case.
