We have investigated the mechanism by which the diameter of solid-state nanopores is reduced by a scanning electron microscope. The process depends on beam parameters such as the accelerating voltage and electron flux and does not involve simple electron-beam-induced deposition of hydrocarbon contaminants. Instead, it is an energy-dependent process that involves material flow along the surface of the nanopore membrane. We also show that pores fabricated in this manner can detect double stranded DNA.