This is 3mm OD Aluminium tube very nice tube BRAND NEW with a wall thickness of 0.45mm, The great thing about this tubing that the next size up(4mm) fits the 3mm inside it  and this is the same for all the tubing by K & S. All the tubing the size below fits inside each other.

This is for a 300mm length

AT3M made by Albion alloys