  In the ultra-thin regime, Bi2Te3 films feature two surfaces (with each
surface being a two-dimensional Dirac-fermion system) with complicated spin
textures and a tunneling term between them. We find in this regime that the
quasiparticle scattering is completely different compared with the thick-film
case and even behaves differently at each thickness. The thickness-dependent
warping effect and tunneling term are found to be the two main factors that
govern the scattering behaviors. The inter-band back-scattering that signals
the existence of a tunneling term is found to disappear at 4 quintuple layers
by the step-edge reflection approach. A four-band model is presented that
captures the main features of the thickness-dependent scattering behaviors. Our
work clarifies that the prohibition of back-scattering guaranteed by symmetry
in topological insulators breaks down in the ultra-thin regime.
