Conductivity of a clean one-dimensional wire

We study the low-temperature low-frequency conductivity a of an interacting one-dimensional electron system in the presence of a periodic potential. The conductivity is strongly influenced by conservation laws, which, we argue, need to be violated by at least two noncommuting umklapp processes to render a finite. The resulting dynamics of the slow modes is studied within a memory matrix approach, and we find an exponential increase as the temperature is lowered, sigma similar to (Delta n)(2)e(T0/(NT)) close to commensurate filling M/N, Delta n = n - M/N << 1, and sigma similar to e((T'0/T)2/3) elsewhere.