Crossover in low-temperature collective spin-density-wave transport

We have measured the nonlinear transport in the spin-density-wave (SDW) state of (TMTSF${)}_2$ ${\mathrm{PF}}_6$. Besides the typical sliding SDW conduction, at temperatures close to ${\mathit{T}}_{\mathrm{SDW}}$, we find a new type of collective electron transport below T≃1 K, with temperature-independent conductivity which follows a Zener-type expression. We discuss the relevant energy scales of the ground-state excitations, both for single-particle tunneling and for coherent SDW processes.