  We discuss the violation of spin-charge separation in generic nonlinear
Luttinger liquids and investigate its effect on the relaxation and thermal
transport of genuine spin-1/2 electron liquids in ballistic quantum wires. We
identify basic scattering processes compatible with the symmetry of the problem
and conservation laws that lead to the decay of plasmons into the spin modes.
We derive a closed set of coupled kinetic equations for the spin-charge
excitations and solve the problem of thermal conductance of interacting
electrons for an arbitrary relation between the quantum wire length and
spin-charge thermalization length.
