We present an expression for the saturated drain current in nanoscale transistors based on multiple reflections of carriers at the virtual source from two adjacent scattering "black boxes". Under certain assumptions and simplifications this new expression reduces to the well known Lundstrom's formula and also to the recent model by Giusi et al. Six macroscopic parameters appear in the 'exact' form of this model. We do not discuss how to derive physical expressions for these parameters. Rather, we emphasize the limitations of Lundstrom's model when applied to nanoscale transistors. Some existing formulae for the carrier backscattering coefficient are examined and compared to our results. We verify our model through a consistency test based on simulation data of a 10 nm gate-length silicon nanowire transistor. (C) 2012 Elsevier Ltd. All rights reserved.