The three-dimensional (3D) topological insulators (TIs)[1,2] are recently discovered solids with strong spin-orbit coupling whose bulk is in a fully gapped insulating phase but surfaces exhibit topologically protected gapless electronic states.It is of crucial importance to investigate the surface transport properties of TIs.However,the reported TIs such as bulk Bi2Se3 and Bi2Te3 are swamped by the larger bulk conduction channels due to their intrinsic defects.Bi2Te2Se,one of ternary tetradymite compounds,has recently been identified to be a 3D TI [3].