"[...] the assimilation in 'width' and 'breadth' consists of devoicing,
though I think there's a lot of variation in what
happens. For examples, the 'Oxford Etymological Dictionary' by Onions gives
final [dT] and [tT] for both of them." Richard WORDINGHAM, ENGLAND
LOL. I can articulate [wIdT]; I have some difficulty articulating [wItT]. I
suppose this is due to the proximity of [t] and [T]. So we could have the
following stages.
1) <wide > wid- + -th> : [wIdT] : [d] is devoiced because [T] is unvoiced.
The result [wItT] can be uttered by a certain number of speakers.
2) [tT] presents difficulties for other speakers, or may sound wrong to
language-conscious speakers, so they dissimilate [t] and [T] by revoicing
[t] into [d] : [widT].