  We report on the electronic structure of $\alpha$-Sn films in the very low
thickness regime grown on InSb(111)A. High-resolution low photon energies
angle-resolved photoemission (ARPES) allows for the direct observation of the
linearly dispersing 2D topological surface states (TSSs) that exist between the
second valence band and the conduction band. The Dirac point of this TSS was
found to be 200meV below the Fermi level in 10-nm-thick $\alpha$-Sn films,
which enables the observation of the hybridization gap opening at the Dirac
point of the TSS for thinner films. The crossover to a quasi-2D electronic
structure is accompanied by a full gap opening at the Brillouin zone center, in
agreement with our density functional theory calculations. We further identify
the thickness regime of $\alpha$-Sn films where the hybridization gap in TSS
coexists with the topologically non-trivial electronic structure and one can
expect the presence of a 1D helical edge states.
