Adding precise z lower limit

paper.tex

@@ -98,6 +98,8 @@
 
 	The integral in~\eqref{eq:chi} picks out values around $u = \frac{c}{\omega} q \sim \frac{1}{z}$.
 	As long as $\frac{1}{z} \ll \frac{\omega}{c} q_{uc}$, the value for the noise will reflect the effects of the superconducting expressions, without picking up the inaccuracies of the transition region.
+	This corresponds with physical distances on the order of $\SI{1}{\nm}$ for metals with $\vf \sim \SI{1e6}{\m\per\s}$.
+	For $z$ smaller than this, more sophisticated dielectric functions would be necessary to take into account inter-atomic spacing.
 
 \section{Experiments \label{sec:experiments}}
 \begin{itemize}