Quasiballistic Transport of Dirac Fermions in a ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ Nanowire

Quantum coherent transport of surface states in a mesoscopic nanowire of the three-dimensional topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ 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.