  Topological insulators are transformative quantum solids with
immune-to-disorder metallic surface states having Dirac band structure.
Ubiquitous charged bulk defects, however, pull the Fermi energy into the bulk
bands, denying access to surface charge transport. Here we demonstrate that
irradiation with swift ($\sim 2.5$ MeV energy) electron beams allows to
compensate these defects, bring the Fermi level back into the bulk gap, and
reach the charge neutrality point (CNP). Controlling the beam fluence we tune
bulk conductivity from \textit{p}- (hole-like) to \textit{n}-type
(electron-like), crossing the Dirac point and back, while preserving the Dirac
energy dispersion. The CNP conductance has a two-dimensional (2D) character on
the order of ten conductance quanta $G_0 =e^2/h$, and reveals, both in
Bi$_2$Te$_3$ and Bi$_2$Se$_3$, the presence of only two quantum channels
corresponding to two topological surfaces. The intrinsic quantum transport of
the topological states is accessible disregarding the bulk size.
