When specifying a movement, specify the easetype as ‘linear’. You will find a list of the easetypes in this answer:
http://answers.unity3d.com/questions/53837/itween-list-of-easetypes.html
And the graphs of each easetype can be fond on this blog page on easing:
http://whydoidoit.com/2012/04/06/unity-curved-path-following-with-easing/