  Topological states of matter challenge the paradigm of symmetry breaking,
characterized by gapless boundary modes and protected by the topological
property of the ground state. Recently, angle-resolved photoemission
spectroscopy (ARPES) has revealed that semiconductors of Bi$_{2}$Se$_{3}$ and
Bi$_{2}$Te$_{3}$ belong to such a class of materials. Here, we present
undisputable evidence for the existence of gapless surface Dirac fermions from
transport in Bi$_{2}$Te$_{3}$. We observe Sondheimer oscillation in
magnetoresistance (MR). This oscillation originates from the quantization of
motion due to the confinement of electrons within the surface layer. Based on
Sondheimer's transport theory, we determine the thickness of the surface state
from the oscillation data. In addition, we uncover the topological nature of
the surface state, fitting consistently both the non-oscillatory part of MR and
the Hall resistance.
  The side-jump contribution turns out to dominate around 1 T in Hall
resistance while the Berry-curvature effect dominates in 3 T $\sim$ 4 T.
