The transport length $l_textrm{tr}$ and the mean free path $l_textrm{e}$ are experimentally determined for bulk and surface states in a Bi$_2$Se$_3$ nanoribbon by quantum transport and transconductance measurements. We show that the anisotropic scattering of spin-helical Dirac fermions results in a strong enhancement of $l_textrm{tr}$, which confirms theoretical predictions cite{Culcer2010}. Despite strong disorder ($l_textrm{e}approx30$~nm), our result further points to the long-range nature of the scattering potential, giving a large ratio $l_textrm{tr}/l_textrm{e}approx8$ that is likely limited by a finite bulk/surface coupling. This suggests that the spin-flip length could reach the micron size in disordered 3D topological insulator nanostructures with a reduced bulk doping, even if due to charge compensation.
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