  The two-dimensional (2D) surface state of the three-dimensional strong
topological insulator (STI) is fundamentally distinct from other 2D electron
systems in that the Fermi arc encircles an odd number of Dirac points. The TI
surface is in the symplectic universality class and uniquely among 2D systems
remains metallic and cannot be localized by (time-reversal symmetric) disorder.
However, in finite-size samples inter-surface coupling can destroy the
topological protection. The question arises: At what size can a thin TI sample
be treated as having decoupled topological surface states? We show that weak
anti-localization(WAL) is extraordinarily sensitive to sub-meV coupling between
top and bottom topological surfaces, and the surfaces of a TI film may be
coherently coupled even for thicknesses as large as 12 nm. For thicker films we
observe the signature of a true 2D topological metal: perfect weak
anti-localization in quantitative agreement with two decoupled surfaces in the
symplectic symmetry class.
