  The Drude-Smith model successfully describes the frequency and phase-resolved
electrical conductivity data for a surprisingly broad range of systems,
especially in the terahertz region. Still, its interpretation is unclear since
its original derivation is flawed. We use an intuitive physical framework to
derive the Drude-Smith formula for systems where microscopically free charges
are accumulated on a mesoscopic scale by localized scatterers. Within this
framework, the model allows us to quantify the microscopic momentum relaxation
time of the charges and the fraction of mesoscopically localized charges in
addition to the direct current limit of the conductivity. We show that the
Drude-Smith model is unique among different Drude-Lorentz models because the
relaxation time of the free carriers also determines the frequency and damping
of the resonance of the bound charges.
