Parabolic heat conduction models are considered. The validity of using a microscopic under harmonic fluctuating boundary heating source is investigated. It is found that using the microscopic parabolic heat conduction model is essential when omegaC(1)/G > 0.1. The phase-shift between the electron gas and solid lattice temperatures is found to be tan(-1) (omegaC(1)/G). This phase-shift reaches a fixed value of 1.5708 rad at very large values of omegaC(1)/G. It is found that using the microscopic parabolic heat conduction model is essential when omega > 1 x 10(9) rad/s for most metallic layers regardless of their thickness.