The quadratic isoperimetric inequality for mapping tori of free
group automorphisms (with M.R. Bridson). Memoirs of the AMS, 203 (2010), no. 955.
If F is a finitely generated free group
and \phi is an automorphism of F
then F \rtimes_\phi Z
satisfies a quadratic isoperimetric inequality.
(This is essentially the two papers with the same title, along with the paper
`Free-group automorphisms, train tracks and the beaded decomposition', turned into a single monograph).