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\sect{The Drude formula}
Consider a ring of length $L$,
with a particle that has the Drude
velocity-velocity correlation function
with a time constant $\tau$.
The temperature is $T$.
\Dn
(a)
Find the conductance of the ring
using the canonical FDT.
\Dn
(b)
What is the conductance if there are $N$ fermions
at zero temperature instead of a single particle.
\Dn
(c)
What is $\tau$, and hence what is the conductance,
if the scattering in the ring is due to
a stochastic segment that has a transmission $g$.