It is a trackway - or tramway if you like. It was used to haul carts up to fetch chalk from the post-medieval chalk and flint quarrying on the hill-top. See 'The Avebury Landscape: Aspects of the field archaeology of the Marlborough Downs' (2005), edited by Graham Brown, David Field and Dave McOmish (Oxbow), especially pages 5 (site plan) and 6; plus of course 'The Creation of Monuments: Neolithic Causewayed Enclosures in the British Isles' (2001) by Al Oswald et al, published by English Heritage.