Matrix notation in ClassicThesis

If bold is out of the question, I might use a script style capital:

\documentclass{article}
\usepackage{amsfonts}
\usepackage{mathtools}

\newcommand\mat[1]{\mathcal{#1}}

\begin{document}
\[
    \mat{A}=\begin{bmatrix}1&2\\3&4\end{bmatrix}
\]

\end{document}