What sets the magnitude of the Hooge noise parameter?

In $S_R(f)/R^{2} = \alpha_H/(N_c f)$, measured $\alpha_H$ spans more than four orders of magnitude between high-quality crystals and disordered films, around the originally proposed universal value 2e-3. For one material class (for example metal films or doped Si), determine how $\alpha_H$ depends on the density and type of mobile defects, whether the noise arises from mobility or carrier-number fluctuations, and derive $\alpha_H$ from a microscopic fluctuator model with independently measured defect parameters.

$\alpha_{\mathrm{H}}$ measured on a sample series with controlled, independently quantified defect density, reproduced by a fluctuator model without free parameters.