  Solid-state nanopores are single molecule sensors that measure changes in
ionic current as charged polymers such as DNA pass through. Here, we present
comprehensive experiments on the length, voltage and salt dependence of the
frequency of double-stranded DNA translocations through conical quartz
nanopores with mean opening diameter 15 nm. We observe an entropic barrier
limited, length dependent translocation frequency at 4M LiCl salt concentration
and a drift-dominated, length independent translocation frequency at 1M KCl
salt concentration. These observations are described by a unifying
convection-diffusion equation which includes the contribution of an entropic
barrier for polymer entry.
