The Landauer-B\"uttiker formalism has been very successful in describing electronic transport in mesoscopic systems. However, unanswered questions remain regarding the role of the exclusion principle, especially if we allow inelastic processes to occur within the device. In this paper we try to answer these questions by starting from the Keldysh formalism and obtaining explicit expressions for the transmission, in terms of microscopic parameters, in the linear-response regime. The transmission is shown to obey reciprocity. The only assumption in this derivation is that the phase-breaking scatterers are assumed to remain in equilibrium and are treated within the self-consistent Born approximation.