  We consider the XXZ quantum spin chain in its massless, disordered regime at
zero field. We derive an exact expression for the two-spinon form-factor of
$S^z=1/2\sigma^z$ by taking a limit of the massive XYZ form-factors found by
Lashkevich and by Lukyanov and Terras. This result is used to find the
two-spinon contribution to the spectral decomposition of the longitudinal
structure factor $S^{zz}(k,w)$. We find that this contribution provides an
accurate approximation to the full structure factor over a wide range of the
anisotropy parameter. The asymptotic behaviour of $S^{zz}(k,w)$ is computed as
the upper and lower $w$ thresholds of the two-spinon $(w,k)$ band are
approached, and an analysis of the region of validity of this threshold
behaviour is performed. Our results reproduce and refine existing threshold
behaviour predictions and extend these results to an accurate description
throughout the two-spinon continuum.
