(quality) is now simply defined by the characteristic scattering time $\tau_t$ ($\tau_q$) as given by the following equations in our leading-order transport theory (i.e., Boltzmann transport plus Born approximation for scattering): $$\begin{aligned}
\frac{1}{\tau_t(k)} = \frac{2\pi}{\hbar} \sum_{\bf k'} \sum_l &
&  \int_{-\infty}^{\infty} dz N_i^{(l)}(z)  \left |u_{\bf k-k'}(z) \right
  |^2 \nonumber  \\ 
& &\times   (1-\cos\theta) \delta(E_{\bf k}-E_{\bf k'}),
\label{eq1}\end{aligned}$$ and $$\begin{aligned}
\frac{1}{\tau_q(k)} = \frac{2\pi}{\hbar} \sum_{\bf k'} \sum_l &
&  \int_{-\infty}^{\infty} dz N_i^{(l)}(z)  \left |u_{\bf k-k'}(z) \right
  |^2 \nonumber  \\ 
& &\times   \delta(E_{\bf k}-E_{\bf k'}),
\label{eq2}\end{aligned}$$ where $N_i^{(l)}(z)$ is the 3D impurity distribution for the $l$-th kind of disorder in the system, and $u_{\bf q}(z)$ is the screened electron-impurity Coulomb interaction given by: $$u_{\bf q}(z) = \frac{V_{\bf q}(z)}{\varepsilon(q)} = \frac{2\pi e^2}{\kappa q}
\frac{e^{-qz}}{\varepsilon(q)},
\label{eq3}$$ with $\varepsilon(q)$ being the static RPA dielectric function for the 2D electron system. We note that $V_q(z) = V(q) e^{-qz} = \frac{2\pi
  e^2}{\kappa q} e^{-qz}$ is simply the 2D Fourier transform of the 3D $1/r$ Coulomb potential, which explicitly takes into account the fact that a spatial separation of ‘$z$’ may exist between the 2D electron layer and the charged impurities. The 2D static RPA dielectric function or the screening function is given by $$\varepsilon(q) = 1 + \frac{2\pi 