By about 1970, it became very rare to see horse riders wearing breeches with flared sides.  Stretch fabrics had come into wide use, so the flexed knee did not need the extra material in order to feel comfortable.  The no-flare style continues to the present.

Most of the law enforcement people who wear breeches and tall boots have adopted the "modern" style.  The only exceptions I know are the Massachusetts and Rhode Island State Police and, of course, the Royal Canadian Mounted Police, with their red coats and full-cut breeches with the yellow side stripe.

As I do not get much of a lift out of this fashion, I only own a couple pairs of no-flare breeches.  This pair is very comfortable.  I have on my Ariat 20" riding boots.

As you can see from this photo, these no-flare, stretch breeches have plenty of "give", probably more than you would ever need.

These are of some stretch corduroy breeches I found in Stockholm some years ago.  These are pretty nice with three zippered pockets, velcro leg closures, and very narrow belt loops.  They have a great fit!