We demonstrate specular electron scattering at single crystal Cu(001) surfaces and quantitatively show how Ta and Ta-oxide overlayers, and adsorbed O₂ molecules perturb the periodic surface potential, causing diffuse electron scattering. These findings are an important step towards narrow low-resistivity interconnect wires, and also provide the basis for a new type of gas sensor. Epitaxial Cu(001) layers with thickness d ranging from 20 nm to 1.4 µm were grown on MgO(001) substrates by ultra-high vacuum magnetron sputter deposition at 80 °C. X-ray diffraction θ-2θ scans, ω-rocking curves, and pole figures show the layers are single crystals with a cube-on-cube epitaxy. In-situ scanning tunneling microscopy shows 20 to 200 nm wide surface mounds with 0.35 to 0.60 nm wide atomically smooth terraces and atomic level roughness. In-situ electron transport measurements at room temperature show a resistivity increase with decreasing thickness, from 1.70±0.06 to 2.65±0.09 μΩ-cm for d = 1400±30 to 20.0±0.4 nm, respectively. The data is consistent with the Fuchs–Sondheimer model and indicates specular scattering at the Cu-vacuum interface with an average specularity parameter p = 0.6±0.1. In contrast, layers measured ex-situ show completely diffuse surface scattering (p = 0) due to sub-monolayer oxidation. Samples that are immersed in liquid N₂ immediately after removal from the vacuum system do not oxidize and exhibit p = 0.7±0.1 at 77 K. In-situ deposition of 0.3 to 7 nm thick Ta barrier layers on Cu(001) films leads to a resistance increase that indicates a transition to completely diffuse surface scattering, independent of the Ta thickness. Oxidation of the Ta layer leads to a partial recovery of the specularity, attributed to a reduced barrier density-of-states at the Fermi-level resulting in a lower scattering probability into barrier-layer states. In-situ exposure of Cu(001) layers to partial O₂ pressures P_ox between 10^-8 and 10^-1 torr cause distinct resistance changes that suggest a multiple step adsorption process: The initially adsorbed O₂ yields a resistance increase due to electronic roughening of the Cu surface, causing complete diffuse scattering at a coverage of 0.14 O₂ monolayers. Continued exposure results in a resistivity drop to the original value, as a complete monolayer is formed that yields specular scattering, followed by an increase associated with diffuse scattering at a chemically oxidized Cu surface. Data analysis provides values for the scattering cross-section of adsorbed molecules and shows that the rate in resistance-change is proportional to P_ox, hence, these layers may become useful as thin metal film gas sensors.