Emergency access conduits were short, narrow access corridors just large enough for an average humanoid to crawl through that were linked to Jefferies tubes in the interior of many Federation starships. The conduits were intended for use only in emergencies to evacuate damaged areas of the ship.
In 2371, while trying to regain access to an inversion field-affected USS Voyager's bridge, Harry Kim thought he remembered seeing an emergency access conduit on the ship's specs which led from the holodeck to deck one behind the bridge. Chakotay shot down the idea, saying heading to bridge on foot would give him a clearer idea of how the ship had been re-oriented. (VOY: "Twisted")