Quasiballistic Transport of Dirac Fermions in a Bi2Se3 Nanowire

Quantum coherent transport of surface states in a mesoscopic nanowire of the three-dimensional topological insulator Bi2Se3 is studied in the weak-disorder limit. At very low temperatures, many harmonics are evidenced in the Fourier transform of Aharonov-Bohm oscillations, revealing the long phase coherence length of spin-chiral Dirac fermions. Remarkably, from their exponential temperature dependence, we infer an unusual 1/T power law for the phase coherence length L-phi(T). This decoherence is typical for quasiballistic fermions weakly coupled to their environment.