  A two-dimensional topological insulator (2DTI) has an insulating bulk and
helical spin-polarised edge modes robust to backscattering by non-magnetic
disorder. While ballistic transport has been demonstrated in 2DTIs over short
distances, larger samples show significant backscattering and a nearly
temperature-independent resistance whose origin is unclear. 2DTI edges have
shown a spin polarisation, however the degree of helicity is difficult to
quantify from spin measurements. Here, we study 2DTI few-layer Na3Bi on
insulating Al2O3. A non-local conductance measurement geometry enables
sensitive detection of the edge conductance in the topological regime, with an
edge mean free path ~100 nm. Magnetic field suppresses spin-flip scattering in
the helical edges, resulting in a giant negative magnetoresistance (GNMR), up
to 80% at 0.9 T. Comparison to theory indicates >98% of scattering is helical
spin scattering significantly exceeding the maximum (67%) expected for a
non-helical metal. GNMR, coupled with non-local measurements demonstrating edge
conduction, thus provides an unambiguous experimental signature of helical
edges that we expect to be generically useful in understanding 2DTIs.
