  We present a series representation for the dynamical two-point function of
the local spin current for the XXZ chain in the antiferromagnetic massive
regime at zero temperature. From this series we can compute the correlation
function with very high accuracy up to very long times and large distances.
Each term in the series corresponds to the contribution of all scattering
states of an even number of excitations. These excitations can be interpreted
in terms of an equal number of particles and holes. The lowest term in the
series comprises all scattering states of one hole and one particle. This term
determines the long-time large-distance asymptotic behaviour which can be
obtained explicitly from a saddle-point analysis. The space-time Fourier
transform of the two-point function of currents at zero momentum gives the
optical spin conductivity of the model. We obtain highly accurate numerical
estimates for this quantity by numerically Fourier transforming our data. For
the one-particle, one-hole contribution, equivalently interpreted as a
two-spinon contribution, we obtain an exact and explicit expression in terms of
known special functions. For large enough anisotropy, the two-spinon
contribution carries most of the spectral weight, as can be seen by calculating
the f-sum rule.
