where $V\left(x\right)$ is the potential and $\psi (t,x)$ is the time- and space-dependent wave function. However, the size of real devices makes it important to derive simplified models that are valid at a larger scale. Typically, one wishes to have kinetic transport equations. As is well-known, this requirement needs one to be able to describe “collisions” between electrons in these devices, a concept that makes sense at the macroscopic level, while it does not at the microscopic (electronic) level. Quantitatively, the question is the following: can one obtain the Boltzmann equation (an equation that describes collisional phenomena) as an asymptotic model for the Schrödinger equation, along the physically relevant micro-macro asymptotics? From the point of view of modelling, one wishes here to understand what are the “good objects”, or, in more technical words, what are the relevant “cross-sections”, that describe the elementary collisional phenomena. Quantitatively, the Boltzmann equation reads, in a simplified, linearized, form :

Here, the unknown is $f(x,v,t)$, the probability that a particle sits at position $x$, with a velocity $v$, at time $t$. Also, $\sigma (v,v^{\text{'}})$ is called the cross-section, and it describes the probability that a particle “jumps” from velocity $v$ to velocity $v^{\text{'}}$ (or the converse) after a collision process.