  We report the experimental observation of Landau quantization of molecular
beam epitaxy grown Sb2Te3 thin films by a low-temperature scanning tunneling
microscope. Different from all the reported systems, the Landau quantization in
Sb2Te3 topological insulator is not sensitive to the intrinsic substitutional
defects in the films. As a result, a nearly perfect linear energy dispersion of
surface states as 2D massless Dirac fermion system is achieved. We demonstrate
that 4 quintuple layers are the thickness limit for Sb2Te3 thin film being a 3D
topological insulator. The mechanism of the Landau level broadening is
discussed in terms of enhanced quasiparticle lifetime.
