  Quantum conductance fluctuations are investigated in disordered 3D
topological insulator quantum wires. Both experiments and theory reveal a new
transport regime in a mesoscopic conductor, pseudo-ballistic transport, for
which ballistic properties persist beyond the transport mean free path,
characteristic of diffusive transport. It results in non-universal conductance
fluctuations due to quasi-1D surface modes, as observed in long and narrow
Bi$_2$Se$_3$ nanoribbons. Spin helical Dirac fermions in quantum wires retain
pseudo-ballistic properties over an unusually broad energy range, due to strong
quantum confinement and weak momentum scattering.
