I have a question, and I'm hoping one of my reader experts might be able to answer it for me. Let me set the stage. One reason 3d topological insulators are a hot topic these days is the idea that they have special 2d states that live at their surfaces. These surface states are supposed to be "topologically protected" - in lay terms, this means that they are very robust; something deep about their character means that true back-scattering is forbidden. What this means is, if an electron is in such a state traveling to the right, it is forbidden by symmetry for simple disorder (like a missing atom in the lattice) to scatter the electron into a state traveling to the left. Now, these surface states are also supposed to have some unusual properties when particle positions are swapped around. These unconventional statistics are supposed to be of great potential use for quantum computation. Of course, to do any experiments that are sensitive to these statistics, one needs to do quantum interference measurements using these states. The lore goes that since the states are topologically protected and therefore robust, this should be not too bad.
Here's my question. While topological protection suppresses 180 degree backscattering, it does not suppress (as far as I can tell) small angle scattering, and in the case of quantum decoherence, it's the small angle scattering that actually dominates. It looks to me like the coherence of these surface states shouldn't necessarily be any better than that in conventional materials. Am I wrong about this? If so, how? I've now seen multiple papers in the literature (here, here, and here, for example) that show weak antilocalization physics at work in such materials. In the last one in particular, it looks like the coherence lengths in these systems (a few hundred nanometers at 1 K) are not even as good as what one would see in a conventional metal film (e.g., high purity Ag or Au) at the same temperatures. That doesn't seem too protected or robust to me.... I know that the situation is likely to be much more exciting if superconductivity is induced in these systems. Are the normal state coherence properties just not that important?