  The Kubo formula for the electrical conductivity is rewritten in terms of a
sum of Drude-like contributions associated to the exact eigenstates of the
interacting system, each characterized by its own frequency-dependent
relaxation time. The structure of the novel and equivalent formulation,
weighting the contribution from each eigenstate by its Boltzmann occupation
factor, simplifies considerably the access to the static properties (dc
conductivity) and resolves the long standing difficulties to recover the
Boltzmann result for dc conductivity from the Kubo formula. It is shown that
the Boltzmann result, containing the correct transport scattering time instead
of the electron lifetime determined by the Green function, can be recovered in
problems with elastic and inelastic scattering at the lowest order of
interaction.
