  We study holographic momentum relaxation in the limit of a large number of
spacetime dimensions D. For an axion model we find that momentum conservation
is restored as D becomes large. To compensate we scale the strength of the
sources with D so that momentum is relaxed even at infinite D. We analytically
obtain the quasi-normal modes which control electric and heat transport, and
give their frequencies in a 1/D expansion. We also obtain the AC thermal
conductivity as an expansion in 1/D, which at leading order takes Drude form.
To order 1/D our analytical result provides a reasonable approximation to the
AC conductivity even at D=4, establishing large D as a practical method in this
context. As a further application, we discuss the signature of the transition
from coherent to incoherent behaviour known to exist in the system for finite
D.
