I would have imagined it is used in the same way it is used for crystals. Mono-crystals are considered mathematically perfect, while poly-crystals (with their g

I would have imagined it is used in the same way it is used for crystals. Mono-crystals are considered mathematically perfect, while poly-crystals (with their grain-boundaries separating different cyrstal orientations) are less perfect. So, for quasi-crystal, the patterning which follows a set of mathematical rules (although lacking translational invariance), can be interrupted, thus being "mathematically less perfect". Now as to why that implied reduced conductivity... thinking in terms of Bloch waves, they require periodicity. In absence of Bloch waves that describe the electron density probability, the electrons are considered to be localized (e.g. as in the Landau level description of electrons implying localization of electrons in a perfect crystal). So for perfect quasi-crystals, this periodicity is fully absent, thus they are localized too. The latter hand-wavy argument is just speculation of mine...