Quantum coherent transport of Dirac fermions in a mesoscopic nanowire of the 3D topological insulator Bi2Se3 is studied in the weak-disorder limit. At very low temperatures, the Fourier transform of Aharonov-Bohm (A-B) oscillations reveals the long phase coherence length of surface states. Remarkably, from the exponential temperature dependence of the A-B oscillations, we infer an unusual 1/T power law for the phase coherence length. This typical temperature dependence indicates the weak coupling of the fermions to the dynamics of their environment and evidences a quasi-ballistic transport regime of spin-chiral Dirac fermions over the perimeter of the Bi2Se3 nanowire (240 nm). This is a signature of the weakening of the transport scattering time for spin-chiral fermions [1].

[1] Dufouleur et al., Physical Review Letters 110, 186806 (2013)