Personal web surfing at work - Allow or Not?

Per research paper Temptation at Work by Harvard Business School, employers should not prohibit the Internet and yet leave it available. Instead, employers should either remove it entirely or, when doing this is impractical, allow employees a certain amount of time for personal Internet activity. Perhaps lunch-breaks can be somewhat shortened to accommodate “surf-time”.

Is the time to replace coffee breaks with internet breaks so employees can do online banking, etc...?  With the new generation of mobile phones & internet access, management control the old way is not possible, and turning off the Internet is not feasible anyway.  The real issue is, how do management motivate employees to remain focus at work?