We report on the magnetotransport in a 90 nm thick n-type GaAs epitaxial thin film in the weak localization ͑WL͒ regime. Low temperature ͑T Յ 50 K͒ magnetotransport data are fit with WL theory, from which the phase coherence time, ϰ T −p ͑p = 1.22Ϯ 0.01͒, are extracted. We conclude that the dominant dephasing mechanism at these temperatures is electron-electron ͑e-e͒ scattering in the Nyquist limit. Evidence of a crossover from two-dimensional to three-dimensional behavior with respect to both coherent transport ͑WL͒ and e-e interactions is observed in the temperature dependence of the zero-field conductivity and , respectively. © 2009 American Institute of Physics. ͓DOI: 10.1063/1.3176968͔Although nanoscale physics in the ballistic regime is an elegant and interesting topic, there are many applications requiring nanoscale devices that retain diffusive transport for application. Examples include devices based on the geometrical enhancement of classical properties such as extraordinary magnetoresistance 1 ͑EMR͒ and more recently extraordinary electroconductance 2 ͑EEC͒ for high resolution magnetic ͑B͒ and electric ͑E͒ field sensing, and the nonballistic spin transistor. 3 Diffusive transport requires that, these devices must have dimensions greater than the elastic mean free path p = ͱ D p , where D is the electron diffusion constant and p is the elastic scattering time. Materials with high room temperature ͑RT͒ mobility ͑ ͒ such as InSb are an obvious choice for high B-field sensitivity ͑Hallϰ , EMR ϰ 2 ͒, 4 but enter the ballistic regime at relatively large dimensions and require demanding fabrication strategies to regain diffusive transport at the nanoscale. 1 Alternatively, materials with lower , such as GaAs, offer the potential for high resolution sensing without the above mentioned difficulties and are also the material of choice for spintronics. 5,6 A thin conducting layer in close proximity to the target medium is advantageous for B-field sensing, e.g., quantum wells or thin epitaxial films. The latter offer advantages in terms of cost and processing complexity and are also applicable to the EEC effect. Accordingly, we report here and quantitatively account for the magnetotransport properties of a thin ͑90 nm͒ n-type GaAs film which exhibits the desirable characteristics noted above.Electron transport in disordered systems has attracted much interest over the past 30 years. [7][8][9] It is an ideal tool for probing electron-electron ͑e-e͒ interactions 10 and weak localization ͑WL͒. 11 Nevertheless, few studies have been reported on three-dimensional ͑3D͒ semiconductor thin films. Savchenko et al. 12 reported on WL in a pre-illuminated GaAs film. Murzin et al. 13 addressed the quantum Hall effect in buried n-GaAs thin films with 3D character, which was described in terms of e-e interactions induced at high B Ͼ 6 T, beyond the WL regime. The sample addressed here differs from those studied previously in that the conducting layer is located at the surface of the structure, preferable for sensing application...