  We show that the spin chain/ladder compound La_3Sr_3Ca_8Cu_24O_41 is an
insulator with hopping transport along the chains. In the temperature range 35
- 280 K, DC conductivity sigma_{DC}(T) follows Mott's law of variable-range
hopping conduction; the frequency dependence has the form sigma(\nu, T) =
\sigma_{DC}(T) + A(T)\nu^{s}, where s \approx 1. The conduction mechanism
changes from variable-range hopping to nearest-neighbor hopping around T_{c}
=300 K. The chain array thus behaves like a one-dimensional disordered system.
Disorder is due to random structural distortions of chains induced by irregular
coordination of the La/Sr/Ca ions.
