  The massless Dirac fermions residing on the surface of three-dimensional
topological insulators are protected from backscattering and cannot be
localized by disorder, but such protection can be lifted in ultrathin films
when the three-dimensionality is lost. By measuring the Shubnikov-de Haas
oscillations in a series of high-quality Bi2Se3 thin films, we revealed a
systematic evolution of the surface conductance as a function of thickness and
found a striking manifestation of the topological protection: The metallic
surface transport abruptly diminishes below the critical thickness of ~6 nm, at
which an energy gap opens in the surface state and the Dirac fermions become
massive. At the same time, the weak antilocalization behavior is found to
weaken in the gapped phase due to the loss of \pi Berry phase.
