Nanoscale systems can ballistically transport electrons and phonons, avoiding any momentum dissipating collisions, because the mean free path of these particles is larger than the dimensions of these systems. We investigate mass flow of molecules in nanoscale channels. The flow is conventionally described by Knudsen theory, which assumes that while inter-particle diffuse (random angle scattering) collisions are absent; the collisions at the confining walls are diffuse. This has been experimentally validated in many systems like silicon nanochannels and zeolites, but not in carbon nanotubes which have shown enormous enhancements from the Knudsen values. In this talk we will investigate the conditions and causes for this enhancement via experiments in angstrom-scale 2-D material assembled channels. We will discover that scattering at the walls can be either diffuse or specular, depending on the fine details of the atomic roughness of the wall, and that quantum effects contribute to the specularity. The dielectric constant of interfacial water has been predicted to be smaller than that of bulk water because the rotational freedom of water dipoles is expected to decrease near surfaces. The exact value is very important for theories describing water-mediated surface interactions in cells, yet experimental evidence is lacking. We measure the local capacitance of water confined in atomically flat 2-D material assembled nanochannels separated by various distances down to 1 nm. Our experiments reveal the presence of an interfacial layer with vanishingly small polarization such that its out of plane ε is only ~2. The electrically dead layer is found to be two to three molecules thick.