A method is provided for fabricating a nanochannel. The method comprises providing a microchannel and controlling collapse of the microchannel so that it collapses to form a nanochannel of desired dimensions. The method employs a collapsible, flexible material such as the elastomer polydimethylsiloxane (PDMS) to form the nanochannel. A master is provided that is configured to have geometric conditions that promote a desired frequency of microchannel collapse. A collapsible material having a stiffness that also promotes a desired frequency of microchannel collapse is molded on the master. The molded collapsible material is removed from the master and bonded to a base, thereby forming the microchannel, which then collapses (or is collapsed) to form the nanochannel of desired dimensions. Nanofluidic and microfluidic devices comprising complex nanochannel structures and micro to nanochannel transitions are also provided.