  A distinguishing feature of Dirac fermions is the Berry phase of $\pi$
associated with their cyclotron motions. Since this Berry phase can be
experimentally assessed by analyzing the Landau-level fan diagram of the
Shubnikov-de Haas (SdH) oscillations, such an analysis is widely employed in
recent transport studies of topological insulators to elucidate the Dirac
nature of the surface states. However, the reported results have usually been
unconvincing. Here we show a general scheme for describing the phase factor of
the SdH oscillations in realistic surface states of topological insulators, and
demonstrate how one could elucidate the Dirac nature in the real experimental
data.
