Spin Conductivity and Spin-Charge Separation in the High $T_c$ Cuprates

We study both the spin and electrical conductivities in models of relevance to the high $T_c$ cuprates. These models describe metallic states with or without spin-charge separation. We demonstrate that, given a linear in temperature dependence of the electrical resistivity, the spin resistivity should also be linear in temperature in the absence of spin-charge separation and under conditions appropriate at least for the optimally doped cuprates, but is in general {\it not} so in the presence of spin-charge separation. Based on these results, we propose to use the temperature dependence of the electron spin diffusion constant to diagnose spin-charge separation in the cuprates.