  Identifying topological insulators and semimetals often focuses on their
surface states, using spectroscopic methods such as angle-resolved
photoemission spectroscopy or scanning tunneling microscopy. In contrast,
studying the topological properties of topological insulators from their
bulk-state transport is more accessible in most labs but seldom addressed. We
show that, in the quantum limit of a topological insulator, the backscattering
between the only two states on the Fermi surface of the lowest Landau band can
be forbidden, at a critical magnetic field. The conductivity is determined
solely by the backscattering between the two states, leading to a resistance
dip that may serve as a signature for topological insulator phases. More
importantly, this forbidden backscattering mechanism for the resistance dip is
irrelevant to details of disorder scattering. Our theory can be applied to
revisit the experiments on Pb$_{1-x}$Sn$_x$Se, ZrTe$_5$, and Ag$_2$Te families,
and will be particularly useful for controversial small-gap materials at the
boundary between topological and normal insulators.
