Fig. S1.

Characterization of the (Bi,Sb)₂Te₃ heterostructure. (A) The hexagonal view of the unit cell of bismuth antimony telluride (Bi,Sb)₂Te₃ consists of perpendicular QLs bound by weak van der Waals interactions. (B) Atomic resolution HRTEM micrograph depicts the QL structure of the TI film (see also ref. 33 for further characterizations). (C) The electronic structure of the (Bi,Sb)₂Te₃ thin film via ARPES. The Dirac cone can be clearly observed from the ARPES intensity map. The Fermi level is indicated by the horizontal dotted line. The position of the Dirac point is indicated by the intersection of the diagonal lines. Magneto-conductance measurements of (Bi_,Sb)₂Te₃ thin films (6 QL) on GaAs substrate at 1.9 K. The external magnetic field is applied perpendicular to the samples. (D) A sharp, negative, weak antilocalization cusp appears under low field, indicating the destructive quantum interference conduction along the topological surface.