Comment on [PRB 91, 165311 (2015)]; Comment on [arXiv:1508.03864]; Comment on [JAP 117, 135102 (2015)]
Yang and Dames [Phys. Rev. B 91, 165311 (2015)] theoretically attributed heating-frequency dependence of apparent thermal conductivity in semiconductors to temporal overlap of the source frequency with phonon scattering rates. They therefore argue that relaxation time (rather than mean free path) forms the natural variable to describe microscale heat conduction. The postulated theory predicts threshold frequencies $\geq$100$\,$MHz for notable conductivity suppression, in poor agreement with 1$-$10$\,$MHz values observed experimentally. Maassen and Lundstrom [arXiv:1508.03864 (2015)] independently obtained near-identical findings through a McKelvey-Shockley flux formalism that they had developed earlier [J. Appl. Phys. 117, 135102 (2015)], and implied that quasiballistic transport can be described by hyperbolic diffusion equations with appropriate boundary conditions. Here, we show that the approximate extension of grey-medium solutions to realistic crystals on which the cited works are built is physically unsuitable for transient quasiballistic heat flow problems. The postulated theories completely overlook the complicated spatial signature of the thermal fields that naturally arises in the proper BTE solutions. A simple two-channel configuration illustrates it is precisely these spatial anomalies that induce heating-frequency dependences. The oversight affects the core methodology of the cited papers and invalidates several key results presented therein. Our analysis reasserts that the studied effect originates in spatial overlap of the source's thermal penetration length with phonon mean free paths, as originally suggested by several prior studies.