  We report the observation of two-dimensional Shubnikov-de Hass (SdH)
oscillations in the topological insulator Tl$_{1-x}$Bi$_{1+x}$Se$_2$. Hall
effect measurements exhibited electron-hole inversion in samples with bulk
insulating properties. The SdH oscillations accompanying the hole conduction
yielded a large surface carrier density of $n_{\rm{s}}=5.1
\times10^{12}$/cm$^2$, with the Landau-level fan diagram exhibiting the $\pi$
Berry phase. These results showed the electron-hole reversibility around the
in-gap Dirac point and the hole conduction on the surface Dirac cone without
involving the bulk metallic conduction.
