  Transport properties of disordered quantum confined helical Dirac systems are
investigated in the large energy limit. As long as the 2D transport length is
larger than the perimeter of the nanowire, the conductance and the Fano factor
are sensitive to disorder only when the Fermi energy is close to an opening of
a transverse mode. In the limit of a large number of transverse modes,
transport properties are insensitive to the geometry of the nanowire or the
nature and strength of the disorder but, instead, are dominated by the
properties of the interface between the ohmic contact and the nanowire. In the
case of a heavily doped Dirac metallic contact, the conductance is proportional
to the energy with an average transmission $\mathcal{T}=\pi/4$ and a Fano
factor of $F\simeq 0.13$. Those results can be generalized to a much broader
class of contacts, the exact values of $\mathcal{T}$ and $F$ depending on the
model used for the contacts. The energy dependence of Aharonov-Bohm
oscillations is determined, when a magnetic flux is threaded through the cross
section of the nanowire.
