  The discovery of the quantum spin Hall effect led to the exploration of the
electronic transport for spintronic devices. Here, we theoretically
investigated the electronic conductance in large-gap realistic quantum spin
Hall system, Pt$_2$HgSe$_3$ nanoribbons. By an ab initio approach, we found
that the edge states present a penetration depth of about $0.9$\,{nm}, which is
much smaller than those predicted in other 2D topological systems. Thus,
suggesting that Pt$_2$HgSe$_3$ allows the exploitation of topological transport
properties in narrow ribbons. Using non-equilibrium Green's functions
calculations, we have examined the electron conductivity upon the presence of
Se\,$\leftrightarrow$\,Hg antistructure defects randomly distributed in the
Pt$_2$HgSe$_3$ scattering region. By considering scattering lengths up to
$109$\,nm, we found localization lengths that can surpass $\mu$m sizes for
narrow nanoribbons ($<9$\,nm). These findings can contribute to further
understanding the behavior of topological insulators under realistic conditions
and their integration within electronic, spintronic devices.
